Dot matrix print heads form a central assembly on which the performance of a dot matrix printer depends. The type of construction and materials employed and the resulting performance characteristics of the print head determine the endurance, and thus the life, of the printer. Dot matrix print heads are typically replaceable in the printer and are accordingly available as spare parts therefor. The performance characteristics of the print head are substantially determined by the system or arrangement of the magnetic circuit, i.e. of the electromagnet-coil/armature system. In this connection a distinction may be drawn between so-called pretensioned magnet systems, in which the armature consists of a spring, and electromagnet-coil/armature systems of the flap armature type.
The present invention relates to the construction and production of either of these two types of armature, although its preferred application--and that to which the present disclosure is specifically directed--is to systems of the flap armature type. In any event, the magnetic induction produced by the electromagnet coil in the magnet core passes in a pulsating manner through the armature. In armatures consisting of a homogeneous ferrous material, the induced voltages thereby produce eddy-current losses since the armature acts in the manner of a short-circuited winding. These eddy-current losses contribute to significant heating of the armature, causing undesired heating of the dot matrix print head and, in addition, to less efficient and effective utilization of energy. This is particularly disadvantageous in that other components or elements present in the dot matrix print head assembly, including the print head itself, are also heated. Not infrequently the resulting temperatures amount to more than 100.degree. C.
It is known that such eddy-current losses can be minimized, or at least significantly reduced, by subdividing the iron body into a multiplicity of thin sheets of material which are insulated from each other (as, for example, by a unilateral paper coating or by surface lacquering) and through the use of silicone-containing iron which has a higher electrical resistance (e.g. alloy dynamo sheets).
When using such laminations on armatures or flap armatures for dot matrix print heads, the relatively small dimensions--such, for example, as an armature length of about 25 mm and an armature thickness of about 6 mm--of the parts necessarily require new manufacturing techniques. Thus, the use of laminated armatures requires a secure connection of the lamellae since the resulting structure, in contradistinction to stationary devices or machines such as transformers or to the stators of electrical machines or of uniformly moving electrical apparati, operatively undergo relatively high forces of acceleration and deceleration in each cycle of armature action.
It has heretofore been proposed, for example in European Patent Application No. A1-0 152 117, that the stacked sheet-metal parts or elements be connected to each other by way of rivets. Because of the small dimensions of the armature assembly, however, rivets are difficult to handle, add excessive weight to the armature and add substantial expense to unit manufacturing costs.